The present invention relates to a process for the distillation of readily polymerizable vinyl aromatic compounds, and more especially, to a process for the vacuum distillation of styrene, substituted styrene, divinylbenzene and polyvinylbenzenes wherein the amount of said materials polymerized during distillation is reduced over an extended period of time, wherein the material accummulating in the bottom or reboiler area of the distillation apparatus is free of material contaminated with sulfur and wherein the rate of throughput for a given distillation apparatus can be increased over the rate at which such apparatus may be operated in accordance with conventional methods.
It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alphamethyl styrene, and divinylbenzene polymerize readily, and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as styrene and divinylbenzene produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation.
In order to prevent polymerization during distillation of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed in connection with prior art distillation processes. For example, common inhibitors useful for inhibiting the polymerization of vinyl aromatics under distillation conditions include 4-tert-butylcatechol (TBC) and hydroquinone. It is preferred, however, to purify vinyl aromatics by using vacuum distillation techniques, whereby these commonly employed inhibitors are rendered unsuitable in view of the fact that they are effective only in the presence of oxygen. The partial pressure of oxygen in a vacuum distillation column is accordingly too low for these conventional inhibitors to be effective. Sulfur is perhaps the polymerization inhibitor most commonly employed to inhibit polymerization of vinyl aromatic compounds during distillation, since sulfur does provide effective inhibition in the absence of oxygen. While sulfur provides a reasonably effective inhibitor, its use in distillation processes results in one very significant disadvantage, namely, there is formed in the reboiler bottoms of the distillation column a valueless waste material which is highly contaminated with sulfur. This waste material furthermore represents a significant pollution or waste removal problem.
Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions, e.g., storage, other purification techniques, etc., for a number of reasons which are not entirely understood in view of the diverse and unpredictable results obtained, only extremely few of these compounds have proved to be of any utility for inhibiting vinyl aromatic polymerization under distillation conditions, particularly under vacuum distillation conditions. In addition, certain compounds which are useful for inhibiting polymerization of one type of vinyl aromatic compound, for example, styrene, have proved to be essentially ineffective for inhibiting polymerization of another species of vinyl aromatic compound, for example, divinylbenzene. A limited number of nitroso compounds have proven to be effective for inhibiting polymerization of styrene monomer during distillation. For example, N-nitroso phenylhydroxylamine and p-nitroso-N,N-dimethylaniline are reasonably effective inhibitors for the distillation of styrene, although they are not particularly soluble in styrene monomer. On the other hand, N-nitroso diphenylamine disclosed in U.S. Pat. No. 3,816,265, assigned to the assignee of the present application has been demonstrated to be a particularly effective polymerization inhibitor under vacuum distillation conditions for both styrene and divinylbenzene, whereas N,N-nitrosomethylaniline as disclosed in U.S. patent application Ser. No. 288,138, also assigned to the assignee of the present application, has been found to be an excellent polymerization inhibitor for styrene under vacuum distillation conditions. One of the most effective inhibitor systems known for divinylbenzene comprises a mixture of sulfur and N-nitroso phenylhydroxylamine.
In a typical distillation process for vinyl aromatic compounds utilizing a polymerization inhibitor, the mixture of vinyl aromatic to be distilled is generally contacted with the chemical polymerization inhibitor prior to being subjected to distillation conditions in the distillation apparatus. It remains as a significant problem today that the amount of polymer formed in the distillation apparatus and in the high purity product recovered therefrom is substantially higher than desired, and occasionally, that complete polymerization occurs inside of the distillation apparatus. For example, in the process of distilling crude divinylbenzene (a mixture containing divinylbenzenes, diethylbenzenes and monovinylbenzenes) to obtain high purity divinylbenzenes, even when inhibited with sulfur and TBC, a divinylbenzene product is obtained which contains significant quantities of polymer which are difficult to separate from the product and are detrimental to the end use of such divinylbenzenes. Furthermore, the material which is removed from the bottom or reboiler area of the distillation apparatus is a highly polluting sulfur-containing waste material which must be disposed of.
Nitrogen oxides are known according to the prior art to be effective for inhibiting polymerization of certain unsaturated compounds, and accordingly, have been employed as polymerization inhibitors in certain types of applications. However, the use of the normally-gaseous nitrogen oxides is predominantly confined to static conditions, e.g., storage, since the use of a gaseous material is strongly suggested against under any conditions where the inhibitor could readily escape. Thus, the use of normally gaseous inhibitors such as the nitrogen oxides has found substantially no application in distillation or similar purification processes involving heat, and this is particularly true in the case of vacuum distillation for the obvious reasons.
Accordingly, there exists a strong need for a polymerization inhibitor which will effectively prevent the polymerization of vinyl aromatic compounds during vacuum distillation thereof.